The present invention relates to a method and apparatus for deposition of particles on surfaces, wherein the particles are provided from an aerosol generation device that regulates the droplet size and concentration provided to the deposition chamber so that precisely sized particles or spheres are deposited on the surface.
Pneumatic atomizers are often used for generating aerosols containing polystyrene latex (PSL) spheres or particles, as well as other particles, For subsequent deposition on substrates, such as semiconductor wafers. The particles are first suspended in liquid such as deionized water to form a suspension. The suspension is then atomized to form droplets. When the droplets evaporate, the PSL spheres or particles become airborne particles. The generation rate of PSL spheres or particles is a function of droplet generation rate of the atomizer and the probability for a droplet to contain PSL spheres or particles.
The droplets produced by a pneumatic atomizer normally have a broad size distribution ranging from less than 0.1 xcexcm to larger than 10 xcexcm. Large droplets have a high probability to contain more than one PSL sphere or particle. If a droplet contains more than one PSL sphere or particle, it is called a multiplet. Multiplets provide more PSL particles than those wanted.
A droplet that does not contain any particles is called an empty droplet. When an empty droplet evaporates, it forms a residue particle resulted from the precipitation of nonvolatile impurities dissolved in the atomizing solution. For example, to prepare a PSL suspension, surfactant is often used to keep suspended PSL spheres from coagulating. The surfactant is one of the sources for residue particles. The size of residue particles depends on the size of the droplets and the concentration of nonvolatile impurities in the atomizing solution. At a given concentration of the nonvolatile impurity, the residue particle size is linearly proportional to the droplet size.
For PSL or particle deposition, the multiplets and the residue particles are always unwanted. Special atomizers will minimize the formation of multiplets and the size of residue particles by removing large size droplets.
The present invention relates to a system for depositing particles on surfaces, in particular semiconductor wafers. The invention insures that there is a minimal amount of unwanted material deposited on the wafer, and that each droplet of the aerosol contains only one sphere or particle. Residues are minimized, and the deposit is uniformly made.
The present invention, in one aspect, provides for an atomizer that will atomize droplets that are only within a particular size range, and will insure that the droplets from the atomizer are of size so they will contain only one particle of the desired material that is going to be deposited. In this way, empty droplets are avoided, and multiplets, that is, a droplet that contains more than one particle, are also avoided.
A differential mobility analyzer, which can be adjusted to emit only the particles that are of proper size, is utilized for insuring one size particle.
Various forms of devices are included for checking the density of the particles in the aerosol and the flow rate. The flow lines permit adding clean gas to the flow of the aerosol as needed, and a pre-deposition sequence permits the aerosol flow to be established at a junction adjacent to the deposition chamber and then switched to the deposition chamber. The procedure reduces the time between deposition cycles.